Amorphous IGZO TFT Degradation Simulation by PBTS(Positive Bias Temperature Stress)

tftex16.in : Amorphous IGZO TFT Degradation Simulation by PBTS(Positive Bias Temperature Stress)

Requires: Victory Device
Minimum Versions: Victory Device 1.14.1.R
In this example, we demonstrate the Positive Bias Temperature Stress simulation due to electron trapping by oxygen related trap. In Victorydevice, we can make an electrochemical reaction formula for explaining the positive threshold voltage shift by electron trapping. This electron trapping state is idenfied as "Oxygen interstitial" which is acceptor-like state when empty. This can be formulated as "O(neutral) + e <=> O-(single ionized negative charged state) or O2(neutral) + 2e- => O2-(double ionized charged state). In this example, we just demonstrate the single ionized charged state. In the other side, we can think of Vo2+ + 2e- <=> Vo reaction, but in this example we omit this electron trapping reaction. So, ionized oxygen vacancy(Vo2+) remains as donor-like trap(NGD).

  • Structure formation using Victorydevice syntax
  • Initial doping concentration of each species for each neutral oxygen and ionized oxygen.
  • Material and model settings for bottom gate passivated IGZO TFT
  • Initial DOS(Density Of State) in a-IGZO bulk.
  • Reaction formula for electron trapping.
  • Initial threshold voltage extraction before applying PBTS.
  • Simulation of Equilibrium concentration.
  • Positive Bias Temperature Stress Simulation.
  • Final threshold voltage shift after applying PBTS.

The bottom gate oxide TFT is initialized by glass substrate thickness of 0.5um. The gate insulator is composed of 0.1um thick nitride(SiNx) and 0.15um thick oxide(SiOx). The IGZO channel thickness is 0.05um and then passivation layer(SiOx) is 0.3um. The gate length is 20um and the device width is 20um.

After constructing bottom gate IGZO TFT structure, we define each ion using "species" command.
Species formula="In:Ga:Zn:O" charge=0 name="O"
Species formula="In:Ga:Zn:O" charge=-1 name="O-"

The formula just define the element composition of IGZO material. In this simulation, each metal cation(In,Ga,Zn) is not reacted with other species. We will use "name" flag to distinguish each species. name="O" define neutral trap state of InGaZnO configuration. The name="O-" defines the ionized charged trap state after reaction.

doping reg=4 species.name="O" uniform conc=1e17
doping reg=4 species.name="O-" uniform conc=1e16

which define the initial concentration of each ion species before applying PBTS.
We also added the interface trap concentration using interface command.

interface s.i x.min=-$l/2-3 x.max=$l/2+3 y.min=-0.36 y.max=-0.34 species.name="O" concentration=2e11
interface s.i x.min=-$l/2-3 x.max=$l/2+3 y.min=-0.36 y.max=-0.34 species.name="O-" concentration=0.2e11

For drift-diffusion simulation of ion during applied bias and temperature, we also need to define the diffusion coefficient as follows.
material material=IGZO species.name="O" species.ea=1.14 species.af=1e7 species.hop=1e-6
material material=IGZO species.name="O-" species.ea=1.14 species.af=1e7 species.hop=1e-6

Next, we define the chemical reaction formula using "reaction" command.

reaction formula="O + e- <=> O-" forward.ea=0.83 forward.rate=1e-7 reverse.ea=0 reverse.rate=1e-7

Probe statement measure the concentration of each ion and the occupation probability during stress time.

probe x=0.0 y=-0.25 material=IGZO ft.species="O" name="ft_O_front"
probe x=0.0 y=-0.25 material=IGZO species.name="O" name="O_front"
probe x=0.0 y=-0.25 material=IGZO ft.species="O-" name="ft_O-_front"
probe x=0.0 y=-0.25 material=IGZO species.name="O-" name="O-_front"

To compare the threshold voltage shift by PBTS, we have simulated the initial threshold voltage curve. After obtaining initial threshold voltage curve, we performed the equlibrium state before actual PBTS. Now, we applied actual positive gate bias, and it continue up to 5e3 sec stress time. At each stress time, the solution structure is saved for later use in order to get final threshold voltage shift.

After obtaining forward reaction simulation, we continue to get a backward reaction(reverse recovery) applying zero voltage at gate.

To control the amount of vth shift and how fast ion react with stress time, user can fit the activation energy(forward.ea/reverse.ea) and reaction coefficient(forward.rate/reverse.rate) in the forward reaction and reverse reaction

This example demonstrate the positive vth shift by forward reaction and negative shift from stressed voltage by reverse reaction.

To load and run this example, select the Load button in DeckBuild > Examples. This will copy the input file and any support files to your current working directory. Select the Run button in DeckBuild to execute the example.